The present invention relates to a doping method to form a region having a desired conductivity type and resistivity in a substrate as part of a process of manufacturing a semiconductor device such as a bipolar transistor or insulated-gate field effect transistor.
One of the conventional typical doping methods is the ion implantation method. In addition, there is another method in which a gaseous impurity element or a compound gas containing the impurity element is applied to a semiconductor surface to form a layer of the impurity element or a layer containing the impurity element on the semiconductor surface before the impurity is diffused into the semiconductor or it is activated by using the formed layer as a diffusion source. Especially, J. Nishizawa et al. Appl. Phys. Lett., 56, 14, P.1334 (1990), "Ultrashallow high doping of boron using molecular layer doping" proposes a molecular layer doping method (hereafter referred to as MLD method) for doping with an impurity capable of forming a very shallow junction and free from shadow effect and physical damage.
However, the conventional ion implantation method has various drawbacks due to its inherent nature as follows:
(i) Damage may be caused on a sample surface due to kinetic energy of impurity ions to be implanted. PA1 (ii) Since the implanted impurity atoms are distributed in a normal distribution having a variance determined by their acceleration energy, it is impossible to form a steep density profile in a deep section. PA1 (iii) With microminiaturation of the semiconductor device, asymmetricity of the device characteristics may be caused due to shadow effects. PA1 (iv) It is not easy to form a shallow junction as channeling would be caused. PA1 (v) If ion implantation is carried out with a reduced acceleration voltage of the impurity ion in order to form a shallow junction, the degree of convergence of the ion beam is degradated, resulting in reduction of production throughput.
In the conventional MLD method, it is easy to form a very shallow junction. However, it is difficult to uniformly dope a plurality of wafers, as described below. FIGS. 2 are process-flow sectional views showing successive stages in a method for doping a plurality of semiconductor wafers with an impurity according to the conventional MLD method. FIG. 2(a) shows the stage of cleaning the surface of semiconductor wafers 1. FIG. 2(b) shows the stage in which a gaseous impurity element 12 or compound gas 12 containing the impurity element is applied to the surface of semiconductor wafers 1 to form an adsorbed layer 13 as a diffusion source of the impurity element or a layer 13 containing the impurity element. Because the feed amount of the gaseous impurity element 12 or the compound gas 12 containing the impurity element depends on the position, or location, at which the wafer is disposed in this process, the thickness and the impurity concentration of the adsorbed layer 13 of the impurity element or the adsorbed layer 13 containing the impurity element is not uniformly formed on the wafers. The thickness and/or the impurity concentration of the adsorbed layer 13 differs from one wafer to another.
FIG. 2(c) shows process stage for diffusing the impurity from the adsorbed layer 13 into the semiconductor wafers 1 and activating the diffused impurity. An impurity diffusion layer 14 is not uniformly formed because its thickness and/or impurity concentration corresponds to the thickness and/or the impurity concentration of the layer 13 of the impurity element or the layer 13 containing the impurity element formed in FIG. 2(b).
As described above, it is difficult to uniformly dope impurity into many wafers by this conventional method.